Comparative techniques in audio mastering

As a Mastering engineer two of the most important considerations working on an album are: ‘Have we improved the aesthetic of the sound overall? Have we taken away or masked anything we liked about the essence of the original session mix?’

To effectively understand these aims it’s important to remember small adjustments in the Mastering scenario will have a large overall affect on the processed audio. For example, if we apply a 1dB boost to our session mix at a given frequency and filter type its equivalent in principle to applying the same filter boost to all the individual tracks in your mix. This could be 24 or more channels - a lot of EQ!

The following series of three comparative techniques help audio engineers to answer these questions truthfully and accurately by interrogating the audio and, importantly, the processing applied.

Also remember we’re Mastering not Mixing. We’re making a session mix that already sounded as the Artist or Producer intended fit for its output format. i.e. Where and on what it is likely to be played back - enabling the mix to translate affectively to its environment. Inherently this means we will make it sound more tonally/spatially balanced and dynamically even – Mastered.

We will make it sound more tonally/spatially balanced and dynamically even – Mastered.

— JP

Equal loudness

The principles of how the human ear reacts to loudness laid out in the work of Fletcher-Munson dictates if we make something louder it will sound fuller; perceptively has more weight or energy. Basically if we turn it up it will sound better! This can be easily proved by placing the same audio on two different tracks and simply turn one up by a couple of dB. Solo between the two and the louder will sound better because our ears are fooling us! Of course we know it’s the same audio, it’s just our perception of the play back.Audio engineers have been aware of this effect for years. From the advent of the first compressors Transfer/Mastering engineers have been trying to make their output format sound louder, initially to improve signal to noise ratio but secondly as it will sound better in comparative playback to another vinyl. This was the case in the early days of the record industry on juke boxes and some radio stations. If you’re record was Mastered louder it would sound better in comparative playback – you can see the appeal to the record companies. This has led to what in recent history has been described as ‘The Loudness Wars’ - but that’s for another discussion.Equally engineers in broadcast initially used limiters to maximise loudness to boost transmission distance and quality but they have always tried to make their program output louder than another’s – again as it will be more catching as it perceptively sounds better. When flicking between stations on the radio the more active or louder stations become more attractive to the ear. Louder adverts on TV make us more aware of their presence and so on.

Even some manufactures have used this affect to enhance their equipments appeal by making the output of a processor slightly louder when all the controls are set flat – hence when turned on and bypassed in/out it makes the sound perceptively better even though it may not be the case, it’s just louder!

In good audio practise when applying processing, especially in Mastering, we need to compare back to the original audio at an equal loudness to hear how the audio has actually been changed. If we boost an EQ we’ve made it louder, hence, almost always irrelevant of the actual change in tone at smaller values (3db and below, 3db being a lot of EQ in a mastering scenario) our ears on comparison without balancing the audio (i.e. turning the EQ boosted audio down) the EQ’d audio will sound better. Equally the opposite would apply if we cut the EQ without boosting the process track the original will sound fuller etc.

In short, when Mastering where generally small adjustments are being applied it is important to compensate for the boost or cut with EQ output level or track gain. Facilitating ‘equal loudness’ in comparison of the processed/unprocessed audio - meaning we can hear the true adjustment in tone.

In short, when Mastering where generally small adjustments are being applied it is important to compensate for the boost or cut with EQ output level or track gain.

— JP

This technique should be applied when processing with EQ and Dynamics or both. Also, comparing the final Mastered file back to the original it is vital to make sure we have answered our main Mastering considerations - ‘Have we improved the aesthetic of the sound overall? Have we taken away or masked anything we liked about the essence of the original session mix?’

Equal loudness should also be used in some parts of the mixing process where we are making more subtle adjustments to a track. If we apply a lot of processing because, let’s say we’re creating a signature sound, using pass filter or just being very corrective it is less important as we can hear how we have changed or improved the sound without the need for this style of comparison.

Sum/Difference

This is an invaluable comparative tool to facilitate understanding how the ‘tool’ applied has change the original audio as we can hear what has been added or subtracted from the overall audio processing.

You can create a Sum / Difference Matrix in any DAW with full delay compensation by creating another track below the original audio – assign this to the same output. Directly copy the audio down on to that track, this has to be sample accurate (use snap if necessary). In some DAW’s you can do all this be simply duplicating the original track.

Polarity invert the new track, (sometimes called phase invert – phase actually means time differential or you could say delay but polarity reverse is the correct term to use in this case) if you don’t have a polarity reverse button on your DAW’s mixer use a plug-in or process the audio offline, making sure you create a new copy or both tracks will have their polarity inverted.

If both tracks are un-muted and you’ve followed the instruction above you’ll hear nothing when the audio is played back. This is because the two audio tracks are in perfect opposite polarity – hence one cancels out the other.

If you insert say an EQ on the process track (top) and restart play back you’ll still hear nothing if the EQ is total passive. If the EQ is affecting the audio in any way you’ll hear what it is adding or subtracting from the audio. Some modelled EQ, if replicating the original unit in detail, will evidence this difference (UAD Pultec or Manley EQ). If we now apply either positive or negative we’ll hear what is being added or taken away by the EQ and the phase of the actual plug-in making the process.

You can use this technique to hear how any process or combinations of processes have changed the sound. This technique is an excellent way to start to understand how a process works i.e. hearing the shape of an EQ, how its 'Q' width changes with increased/decreased gain and so on.

If you’re using external analogue hardware in the chain you can ‘ping’ the external effects loop in some DAW’s to achieve the correct delay compensation or just record the process back in and manually line-up.

In practise now we have a processed track on top and an original audio track below (polarity reversed) we can, by muting just the bottom track, listen and work on processing the audio. Un-muting the bottom track we will hear the difference – the effect of the processing applied. By soloing the bottom track we’ll hear the original audio in comparison (assuming your audio program is set to defeat mute on a soloed track).

Remember – louder sounds better to our ears - you must make equal loudness comparisons to hear the true change in the audio when comparing between the original and processed. If you’ve readjusted level to make an equal loudness comparison to hear the Sum/Difference you must reset both audio tracks to the same output level.

Remember – louder sounds better to our ears - you must make equal loudness comparisons to hear the true change in the audio when comparing between the original and processed.

— JP

You will often find a Sum/Difference matrix built in to plug-ins, such as noise reductions were it can be often useful to fully perceive what has been removed to decide on how much reduction to apply. Equally in Mastering it’s an important interrogation tool.

I use this technique every time I’m mastering a track or album – it makes me aware of how the processing is effecting the overall sound. Is there anything being removed or enhanced I didn't what to and most importantly the question we should always ask ourselves – have we actually made the audio sound better? If we have not improved the sound overall then we have not done our job.

Mid/Side exploration

M/S is a powerful corrective tool in the Mastering engineer’s arsenal when it comes to correcting mix errors. Equally, when used as a comparative tool we can fully interrogating the stereo/mono components of a mix. When listening to a track for the first time I’ll always listen to the (M) Mid (L+R – 6db) and the (S) Side (L-R – 6db). A good Mastering monitor controller will do this formula for you or you can use a plug-in tool such as Voxengo MSED or BXsolo (both freeware).

By listening to the ‘S’ in isolation we can establish the stereo elements of a mix and what can be enhanced to improve the overall image. Often less well mixed tracks will lack width or have ambience issues that can be greatly improved by the judicious use of EQ, and pre-emphasis at the Mastering stage.

In short, the use of M/S as a listening tool will enhance your ability to understand the session mix panorama, its elements and help you to improve the sound imaging/clarity overall.In conclusion

Basically we need to make sure we’re not being fooled buy our ears. They’re our best and should be our most trusted tool though they can be our worst enemy without the application of good audio practice.

Every time I Master an album I’ll use all these comparative tools to help my abilities to creatively enhance the audio. If you start to integrate this good practice in to your daily engineering you will not only improve your understanding of sound as an engineer but start to produce more consistence, better sounding mixes and masters. Though always remember - sometimes no process can be the best process. The use of comparative tools in practice will help you to truly answer our main aims of a Mastering engineer - ‘Have we improved the aesthetic of the sound overall? Have we taken away or masked anything we liked about the essence of the original session mix?’